Metal oxide varistor with selectively positionable intermediate electrode

ABSTRACT

A metal oxide varistor is disclosed formed of a varistor body having an alpha in excess of 10 in the current density range of from 10 3 to 102 amperes per square centimeter. First and second electrodes are laterally spaced along the varistor body, and a third electrode is located between and spaced from the first and second electrodes. The third electrode may be slidably mounted on the varistor body or may be in the form of discrete, spaced elements fixedly attached to the surface of the varistor body.

United States Patent 1191 Harnden 1 METAL OXIDE VARISTOR WITH SELECTIVELY POSITIONABLE INTERMEDIATE ELECTRODE Inventor: John D. I-Iarnden, Schenectady,

Assignee: General Electric Company, Syracuse, N.Y.

Filed: Oct. 13, 1971 Appl. No.: 188,984

U.S. Cl. 338/20, 338/92 Int. Cl H0lc 7/10 Field of Search 338/20, 21, 13, 52

References Cited UNITED STATES PATENTS 3,6l 1,073 10/1971 Hamamoto 338/20 1451 Aug. 21, 1973 2,935,712 5/1960 Oppenheim et al. 338/20 3,456,228 7/l969 Wright 338/92 X Primary Examiner-C. L. Albritton Attorney-Robert J. Mooney et al.

[ 5 7] ABSTRACT 4 Claims, 7 Drawing Figures METAL. oxIDE VARISTOR wITII SELECTIVELY POSITIONABLE INTERMEDIATE ELECTRODE My invention is directed to a metal oxide varistor having at least three electrodes with the third electrode being located to allow selection of a desired potential as compared to the first electrode.

It can be generally stated that the current which flows between two spaced points is directly related to the potential difference between the points. For most known substances current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohms law-to be its resistance. There are, however, a few known substances which have been observed to exhibit non-linear resistances and which require resort to the following equation (I) to relate quantitatively current and voltage:

: V/C)alphu where V is the voltage between two points separated by a body of the substance under consideration, I is the current flowing between the two points, C is a constant, and alpha is an exponent greater than I. There are manyknown electrical circuits in which it is quite desirable to incorporate one or more functional elements having non-linear or exponential resistance characteristics. For example,'the non-linear resistance properties of silicon carbide have been widely utilized in commercial silicon carbide varistors. Typically silicon carbide varistors exhibit an alpha of no more than 6.

It has been recently appreciated that varistors having alphas in excess of within the current density range of 10' to 10 amperes per square centimeter may be made from bodies comprised or metal oxides. The metal oxide body may be formed predominantly of zinc oxide with small quantities of one or more other metal oxides also being present. Metal oxide varistors having alphas in excess of 10 are disclosed in Canadian Pat. No. 831,691, issued Jan. 6, 1970, for example. While the alphas of these metal oxide varistors is identified by the current density range of 10' to 10 amperes per square centimeter, which characteristically exhibits substantially constant alphas, it is appreciated that the alphas remain high also at higher and lower current densities, although some decline from maximum alpha values have been observed.

Certain improved forms of metal oxide varistors are disclosed in my co-pending applications Ser. No. 165001 METAL OXIDE VARISTOR WITH LATER- ALLY SPACED ELECTRODES, filed July 22, 1971, and Ser. No. 169220 HYBRID CIRCUIT ARRANGE- MENT WITH METAL OXIDE VARISTOR SHUNT, filed Aug. 5, 1971.

It is one object of my present invention to provide a metal oxide varistor having a third electrode which is capable of selectably dividing the potential difference between two remaining electrodes. It is another object to provide a three electrode metal oxide varistor in which the third electrode may be selectively located with respect to a remaining electrode. It is still another object to provide a three electrode metal oxide varistor in which the third electrode is in the form of discrete elements differentially spaced from one electrode so that a desired potential relationship between the one electrode and one of the elements of the third electrode can be established.

In one aspect, my invention is directed to the combination comprising a metal oxide varistor body means having an alpha in excess of 10 in the current density range of from 10* to 10 amperes per square centimeter. A first electrode adapted to be biased to a first potential lies in ohmic contact withthe metal oxide varistor body means at a first location. A second electrode adapted to bebiased to a second potential lies in ohmic contact with the metal oxide varistor body means at a second location spaced from the first location. Third electrode means lies in ohmic contact with the metal oxide varistor body meansfor permitting selectable potentials intermediate the fii'st arid second potentials to be picked from the metal oxide varistor body means at any one of aplurality of selectable locations spaced from and mediate the first and second locations.

My invention may be better understood by reference to the following detailed description considered in conjunction with the drawings, in which FIG. 1 is an elevation of one metal oxide varistor formed according to, my invention;

FIG. 2 is a sectional view taken along section line 2-2 in FIG. 1;

FIG. 3 is a plan view of second embodiment of my invention;

FIG. 4 is a sectional view takenfalon g section line 4- 4 in FIG. 3; g

FIG. 5 is a schematic circuit diagram;

FIG. 6 is a plan view of a third embodiment of my invention; and r FIG. 7 is a plan view of a fourth embodiment of my invention. I

In FIGS. 1 and 2 a varistor is shown formed according to my invention including a metal oxide varistor body 102, which has an alpha as defined in equation (1 in excess of 10. The metal oxide varistor body may be formed according to the teaching of the Canadian patent cited above or in any other known manner. As shown, the body is in the geometrical configuration of a cylindrical rod. First and second electrodes 103 and 104 are located at opposite ends of the cylidrical rod. The electrodes may be ohmically conductively associated with the varistor body in any convenient conventional manner.

Surrounding the rod is a third electrode 106. In the specific form shown the third electrode is in the form of a resilient band. The opposite ends of the band are separated by a gap 108 The band is normally resiliently biased so that it compressively engages the surface of the rod. In this way the band is fixedly positioned between the electrodes 103 and 104 in predetermined spaced relation thereto. In order to reposition the band it is merely necessary to apply a spreading force to the opposite ends thereof so as to enlarge the gap 108. This increases the diameter of the band and allows it to be slidably repositioned along the rod to any desired spacing with respect to the first and second electrodes.

In FIGS. 3 and 4a varistor 200 is shown, which is an alternative embodiment of my invention. A metal oxide varistor body 202 is supported on a dielectric substrate 204. First and second electrodes 206 and 208 are ohmically conductively attached to the metal oxide varistor body at its opposite ends along a first major surface 210 thereof. A third electrode means is formed by a plurality of discrete electrode elements 212a, 212b, 212e,

212d, 212e, 2l2f, and 212g positioned along the first major surface in ohmic contact with the metal oxide varistor body. Each third electrode element is spaced both from the first and second electrodes and from each remaining element of the third electrode means. The third electrode elements are positioned so that in order of progressive displacement from the first electrode every other (meaning every second or alternate) element is also laterally offset. In this way a larger number of elements may be located within a given lateral displacement along the major surface of the metal oxide varistor body than could be achieved if the elements were all aligned. In the specific embodiment shown the edge of the element 212a most remote from the electrode 208 is aligned with the edge of the next successive element 212b, which (in terms of displacement distance from the electrode 208) is closest to the element 212a. In like manner each of the successive elements is provided with an edge surface nearest the electrode 208 that is aligned with the edge surface most remote from the electrode 208 of the next nearest element to the electrode 208. In this way any lack of selectivity in spacing the elements with respect to the electrode 208 attributable to the requirement of laterally spacing the elements from each other is entirely avoided. In other words, it is apparent that spacing of adjacent elements is achieved through the use of lateral offsets rather than by preventing elements from lying at certain distances from one of the first and second electrodes.

The metal oxide varistors 100 and 200 may be utilized in like or similar manner. In utilizing the metal oxide varistor 100 the first and second electrodes l03 and 104 may be electrically biased to lie at differing potentials, so that a potential difference is established between the electrodes. The current flowing between the electrodes through the metal oxide varistor body 102 for a given potential difference between the electrodes will be a function of the conductivity of the metal oxide varistor body when subjected to that particular level of electrical stress. The conductivity is a function both of A the composition of the metal oxide forming the body and of the distance between'the first and second electrodes. Hence, referring to equation (1 it can be seen that with a large spacing between the first and second electrodes the current density will be small, since the value of C will be comparatively large. In this way a relatively large potential difference may be present across the first and second electrodes with only a small current flowing therebetween. However, it should be borne in mind that the current and voltage relationship is still controlled by equation l hence any further increase in the potential will be accompanied by a disproprotionate increase in the current, attributable to the high alpha of the metal oxide varistor body.

I have recognized that even though current and voltage are not linearly related by reason of the large alpha of the metal oxide varistor body, there will be a linear voltage drop across the length of the body. In order to place the third electrode at any desired potential intermediate the potentials of the first and second electrodes it is merely necessary to position the third elec trode at an intermediate location which is readily ascertainable by reason of the linear potential drop. As a specific example, if it is assumed that the first and second electrodes lie at 100 and 200 volts, respectively, the third electrode may readily be adjusted to a potential of 140 volts merely by locating the edge of the third electrode nearest the first electrode so that 40 percent of the length of the metal oxide varistor body lying between the first and second electrodes lies between the first and third electrodes.

From the foregoing it is apparent that the third electrode 106 of the varistor may be positioned at a selected potential intermediate the potentials of the electrodes 103 and 104 merely by expanding the band and sliding the electrode along the rod forming the metal oxide varistor body. When the expanding forces are removed from the band, its resilience will firmly locate the band in the desired position on the rod.

In using the varistor 200 it is apparent that when the first and second electrodes 206 and 208 are biased to different potential levels, the third electrode means will assume a potential intermediate the applied potentials. While the third electrode means is comprised of elements 212a-212g each of which are fixedly positioned with respect to the first and second electrodes, it is apparent that by reason of their differing spacings with respect to the first and second electrodes each of the elements of the third electrode will lie at a different potential level. Accordingly, an intermediate potential may be selected merely by selecting one of the third electrode elements that lies at this potential. Whereas in the varistor 100 a continuous spectrum of intermediate potentials may be selected, in the varistor 200 the selectable intermediate potentials are in the form of a plurality of stepped increments. The difference in value of the increments is noted to be a function merely of the width of the elements. If only one row of elements were employed, rather than two laterally offset rows as shown, the stepped increments of potential would be a function both of the width of the elements and of their spacing.

It is apparent that the varistors formed according to my invention are readily applicable to a variety of uses by reason of their capability for selective electrode spacings. In a very simple application only the first and third electrodes may be utilized. The spacing between the first and third electrodes may be selectively chosen to give the desired current and voltage characteristics for the device, and the device may be substituted for any conventional two terminal varistor. In a more preferred application the first electrode may be biased to a potential differing from that of the second electrode, and an intermediate potential of a selectable value may be picked from the varistor body by the ohmic connection provided by the third electrode. In still other applications the third electrode may be biased to a potential either higher or lower than that of either the first and second electrodes.

To provide a specific example of an application for a varistor according to my invention, in FIG. 5 an electrical circuit 300 is schematically shown comprised of input terminals 302 and 304 which may be attached to an alternating current or direct current potential source, not shown. An electrical load 306 and a triac 308 are connected in series across the input terminals. Connected to the gate lead of the triac is a diac 310. A capacitor 312 is connected between the input terminal 304 and the diac.

A varistor 314, which may be either varistor 100 or varistor 200, is connected across the input terminals of the circuit. The varistor is preferably connected across the input terminals by what correspond to the first and second electrodes in the varistors 100 and 200. The varistor provides a selectably referenced third electrode which is connected to the diac and capacitor.

When the input terminals 302 and 304 of the circuit are connected to an A-C or pulsating D-C potential source, proper spatial location of the third electrode with respect to the first and second electrodes can control the phase angle at which the triac is fired. As is well understood in the art, firing of the triac is achieved by firing the diac 310. The diac switches from an initially high impedance to a comparatively low impedance state when a predetermined potential difference thereacross is exceeded. The relationship of diac firing to application of a potential difference across the input terminals of the circuit is a function of the rate at which the capacitor 312 is charged. The potential difference between electrode of the varistor 314 connected to the input terminal 302 and the third electrode controls the rate at which the capacitor can be charged. Referring to equation (1), it can be seen that at the beginning of a voltage half-cycle only a low charging current may be permitted to flow by the varistor. However. since the current increases as a function of the voltage raised to at least the tenth power, the minimum contemplated value of alpha, it can be seen an increasing potential difference between the third electrode and input terminal 302 greatly increases the charging rate of the capacitor. Hence a much more selective charging of the capacitor is obtainable than is possible merely using a linear resistor as a current limiter. A second function that is provided by the varistor 314 is to selectively shunt the load and triac when a large potential is placed' across the input terminals. Thus, the varistor functions very effectively not only to allow selective phase control of triac firing, but also to protect the circuit against voltage transients.

The varistor 314 is unique in that it performs a circuit function never before performable with a single electronic component. Consider, for example, the result of substituting a conventional resistive potentiometer for the varistor 314. An increase in voltage applied across the terminals 302 and 304 would result in a proportionate acceleration of the rate at which the capacitor 312 is charged with a corresponding decrease in the phase angle of firing. With the varistor 314 the third electrode is for all practical purposes clamped at a fixed maximum potential difference with respect to the input terminal 302, since the input terminals 302 and 304 are also effectively clamped at a fixed maximum potential difference thereacross. Hence, the potential applied across the input terminals of the circuit may vary above a predetermined potential in an unpredicted manner without having any appreciable effect on the phase angle of firing. In fact, the varistor 314 performs in a manner that could only be approximated with a resistive potentiometer which is connected across the input terminals of the circuit in parallel with a pair of backto-back Zener diodes. It is, of course, recognized that the relative invariancy with which the input potential can be clamped improves as the alpha of the varistor increases. Since the varistor 314 exhibits an alpha in excess of 10, it is capable of clamping the input potential to the circuit with a high degree of effectiveness.

In FIG. 6 a varistor 400 is formed according to my in- .vention comprised of a metal oxide varistor body 402.

On the major surface of the body exposed to view are located a first electrode 404 and a second electrode 406. The electrodes may be identical to electrodes 206 and 208. The first electrode presents an edge surface 408 that is spaced from and parallel with an edge surface 410 of the second electrode. Thus, when the first and second electrodes are biased to differing poten tials, a potential drop is present within the metal oxide varistor body that is uniform across the width of the varistor. A third electrode 412 is located in ohmic conductive relation with the exposed surface of the metal oxide varistor body between the first and second electrodes. The third electrode is located so that it presents an edge 414 that diverges across the width of varistor from the edge 410 of the second electrode.

Initially the varistor 400 is formed so that the third electrode is a unitary element lying closer to the second electrode at its nearest approach than is deemed desirable in any contemplated application for the varistor. Also, the unitary third electrode at the greatest divergence of its edge 414 from the edge 410 of the second electrode lies at a greater distance than is deemed desirable for any contemplated application. The varistor may then be precisely adjusted to yield a specific voltage and current relationship between the second and third electrodes merely by dividing the third electrode into two segments 416 and 418. The nearest approach of the segment 418 to the edge 410 is precisely chosen to yield the desired voltage and current characteristics when a potential difference is established between the first and second electrodes and a lead attachment is made to the segment 418. The third electrode may be conveniently segmented using techniques conventionally employed for trimming resistors in thick film integrated circuits.

. In FIG. 7 a varistor .500 is illustrated which may be identical to varistor 400, except that in place of the continuously diverging edge 414 associated with the third electrode a stepped edge 502 is provided. Each step along the edge is preferably substantially parallel to the edge of the adjacent electrode 504. The advantage of using a stepped edge is that a sharper currentvoltage knee or switching characteristic is obtained than is obtainable using a diverging edge.

In the foregoing description I have set forth certain aspects of my invention by reference to certain specific embodiments. It is appreciated that it may be desired to modify or add to the specific embodiments shown to serve specific end applications. For example, none of the varistors above shown are packaged in any way. My varistors may be utilized without packages in protected environments. For example, in a controlled inert gas atmosphere there would be no reason to package my varistors. In many applications it will be desirable to hermetically encaseat least the metal oxide varistor body portion of my varistors. This may be accomplished by encapsulating the varistors in a dielectric such as glass or plastic, for example. Alternatively, the varistors may be fitted into hermetically sealed housings in generally similar manner as semiconductive elements are so housed. When the varistor 200 is encapsulated, it is preferred that all of the third electrode elements protrude beyond the encapsulant to allow selection of one element after packaging.

I have shown the metal oxide varistor body in the form of a rod and in the form of a supported element. The exact geometrical configuration of the metal oxide varistor body is not deemed critical to my invention. For example, the use of a slidable electrode is possible even with flat plate metal oxide varistor bodies. Generally flat plate metal oxide varistor bodies are preferred where the varistor is to be used with or form a part of an integrated circuit. No particular means have been shown for attaching electrical connectors to the electrodes, since this is considered to be well within the ordinary skill of the art. It is noted that a number of mechanical arrangements are known to the art for slidably mounting electrodes that could be utilized in the prac-. tice of my invention without the exercise of invention. For example, instead of forming the entire band 106 of metal only one specific point on the band may be allowed to make conductive contact with the metal oxide varistor body. For many applications this may allow more accurate choice of the exact potential to be picked off by the third electrode. Instead of having the slidable third electrode make contact directly to the metal oxide varistor body directly it may make contact to metallized surface areas of the metal oxide varistor body. For example, in the varistor 100 there may be provided a plurality of metal rings extending around the rod and spaced laterally along the rod between the first and second electrodes. The rings may be fixedly attached to the metal oxide varistor body in ohmic conductive relation therewith. The resilient band 106 would then slide over the rings and make electrical connection to whichever ring or rings it happened to overlie.

In the discussion of my invention I have made reference to only one electrode lying between the first and second electrodes, It is recognized that in various applications it may be desired to make electrical connection to more than one location along the metal oxide varistor body between the first and second electrodes. For example, it may be desired to pick-off from the metal oxide varistor body two or more potential levels which are intermediate the potentials of the first and second electrodes. In the varistor 100 this may be accomplished merely by adding one or more additional resilient bands. In the varistor 200 this may be accomplished merely providing electrical connections to more than one of the third electrode elements. In the varistors 400 and 500 this may be accomplished by forming plural discrete third electrode segments that each will lie at the desired intermediate potentials.

Inasmuch as numerous modifications of my invention will readily occur to those skilled in the art, it is intended that the scope of my invention be determined by reference to the following claims:

I claim: 1. The combination comprising metal oxide varistor body means having an alpha in excess of 10 in the current density range of from 10' to l0 amperes per square centimeter, afirst electrode adapted to be biased to a first potential lying in ohmic contact with said metal oxide varistor body means at a first location,

a second electrode adapted to be biased to a second potential lying in ohmic contact with said metal oxide varistor body means at a second location spaced from said first location, and

third electrode means lying in ohmic contact with said metal oxide varistor body means for permitting selectable potentials intermediate said first and second potentials to be picked from said metal oxide varistor body means, said third electrode means being formed of a resilient band, said band being resiliently biased to compressively engage said varistor body means.

2. The combination according to claim '1 in which said resilient band is movable with respect to said metal oxide. varistor body means. 1

3. The combination according to. claim 1 in which said resilient band has first and second ends, said first and second ends being separated by a gap, said gap allowing said band to be disengaged from compressive engagement with said varistor body means by application of a spreading force to enlarge said gap so that said band is slidably repositionable along said varistor body means.

4. The combination according to claim 1 in which said metal oxide varistor body means is a rod. 

2. The combination according to claim 1 in which said resilient band is movable with respect to said metal oxide varistor body means.
 3. The combination according to claim 1 in which said resilient band has first and second ends, said first and second ends being separated by a gap, said gap allowing said band to be disengaged from compressive engagement with said varistor body means by application of a spreading force to enlarge said gap so that said band is slidably repositionable along said varistor body means.
 4. The combination according to claim 1 in which said metal oxide varistor body means is a rod. 